Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits.

It is demonstrated by means of density functional and ab initio quantum chemical calculations, that transition-metal-carbon systems have the potential to enhance the presently available area density of magnetic recording by 3 orders of magnitude. As a model system, Co2 benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy of the order of 0.1 eV per molecule, large enough to store permanently 1 bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite.

Transition metal dimers as potential molecular magnets: A challenge to computational chemistry.

Dimers are the smallest chemical objects that show magnetic anisotropy. We focus on 3d and 4d transition metal dimers that have magnetic ground states in most cases. Some of these magnetic dimers have a considerable barrier against re-orientation of their magnetization, the so-called magnetic anisotropy energy, MAE. The height of this barrier is important for technological applications, as it determines, e.g., the stability of information stored in magnetic memory devices. It can be estimated by means of relativistic density functional calculations. Our approach is based on a full-potential local-orbital method (FPLO) in a four-component Dirac-Kohn-Sham implementation. Orbital polarization corrections to the local density approximation are employed. They are discussed in the broader context of orbital dependent density functionals. Ground state properties (spin multiplicity, bond length, harmonic vibrational frequency, spin- and orbital magnetic moment, and MAE) of the 3d and 4d transition metal dimers are evaluated and compared with available experimental and theoretical data. We find exceptionally high values of MAE, close to 0.2 eV, for four particular dimers: Fe(2), Co(2), Ni(2), and Rh(2).

Half metals: from formal theory to real material issues.

At the basic level of collinear spin density functional theory, half metallic ferromagnets represent a fundamentally different state of matter: for low-energy physics the spin degree of freedom is absent, although the system is spin polarized. This makes such systems highly attractive for spintronics applications, but also introduces fundamental new phenomena such as a superconducting state in which the concept of 'spin-pairing' never appears. A fully relativistic theory introduces spin-orbit coupling and destroys the precise aspect of half metallicity; does this make 'half metals' a half truth? Obviously not in any real sense: spin-orbit coupling arises as a perturbative effect, and although necessitating reconsideration from the formal viewpoint, leaves half metallicity as a qualitatively distinct state. We provide a simple model that suggests that in appropriate circumstances this qualitative distinction may even survive strong spin-orbit coupling.

Nuclear magnetic resonance at up to 10.1 GPa pressure detects an electronic topological transition in aluminum metal.

High-sensitivity (27)Al nuclear magnetic resonance (NMR) measurements of aluminum metal under hydrostatic pressure of up to 10.1 GPa reveal an unexpected negative curvature in the pressure dependence of the electronic density of states measured through shift and relaxation, which violates free electron behavior. A careful analysis of the Fermiology of aluminum shows that pressure induces an electronic topological transition (Lifshitz transition) that is responsible for the measured change in the density of states. The experiments also reveal a sudden increase in the NMR linewidth above 4.2 GPa from quadrupole interaction, which is not in agreement with the metal's cubic symmetry.